The present invention is directed toward the synthesis of sialyl.alpha.2.fwdarw.3.beta.galactoside compounds. Exemplary natural materials having the contemplated group are the gangliosides G.sub.M3, G.sub.M2, G.sub.M1 and G.sub.D1a, and the sialyl Lewis ligands, sialyl Lewis.sup.x and sialyl Lewis.sup.a as are present in leukocyte and non-leukocyte cell lines that bind to receptors such as the ELAM-1 and GMP 140 receptors. Polley et al., Proc. Natl. Acad. Sci., USA, 88:6224 (1991) and Phillips et al., Science, 250:1130 (1990).
A contemplated sialyl.alpha.2.fwdarw.3.beta.galactoside can be prepared using well known organic chemical methods. Several sialyl.alpha.2.fwdarw.3.alpha.galactosides have also been prepared using enzymatic techniques. The organic chemical techniques are typically more cumbersome than the enzymatic techniques, requiring many protection and deprotection steps, whereas enzymatic techniques do not.
In the application of enzymes to the field of synthetic carbohydrate chemistry [(a) Toone et al., Tetrahedron, 45:5365 (1989); (b) David et al., Adv. Carbohydr. Chem. Biochem, 49:175 (1991); (c) Drueckhammer et al., Synthesis, 499 (1991); (d) Ichikawa et al., Anal. Biochem., 202:215 (1992)], the use of sialyltransferase [(a) Sabesan et al, J. Am. Chem. Soc., 108:2068 (1986); (b) Palcic et al., Carbohydr. Res., 190:1 (1989); (c) Srivastava et al., Carbohydr. Res., 207:259 (1990); (d) Palcic et al., Glycobiology, 1:205 (1990); (e) Pozsgay et al., J. Org. Chem., 56:3377 (1991); (f) Ichikawa et al., J. Am. Chem. Soc., 113:4698 (1991); (g) Ito et al., J. Am. Chem. Soc., 115:1603 (1993)], for enzymatic sialylation is recognized to offer advantages over chemical methods [Reviews: (a) Okamoto et al., Tetrahedron, 46:5835 (1990); (b) Deninno, Synthesis, 583 (1991)], due to the virtually complete stereoselectivity and linkage specificity offered by the enzymes [Ito et al., Pure Appl. Chem., 65:753 (1993)]. However, a major drawback of enzymatic of enzymatic sialylation in general is the rather strict acceptor substrate specificity of these enzymes that permits the synthesis of only a limited number of sialoside sequences.
A novel enzymatic process to introduce sialic acid (NeuAc) that addresses the above limitation and is widely applicable to the synthesis of glycan chains containing the terminal NeuAc.alpha.2.fwdarw.3Gal sequence such as sialyl Lewis .sup.x and its analogues that inhibit binding by the ELAM-1 and GMP-140 receptors is disclosed hereinafter.